SDRPlay in gr-osmosdr

Here is a PyBOMBS recipe file that pulls gr-osmosdr from my sdrplay2 branch. If anyone would like to try out this support with a PyBOMBS build, this would be the easiest way. Best to build everything from scratch, but you can remove the src/gr-osmosdr directory, do a pybombs fetch/rebuild if you're comfortable with that. The SDRplay SDK v2.11 is required, and should be installed before gr-osmosdr is rebuilt. The ENABLE_NONFREE flag is turned on for this build. This is just for testing, so I can get some feedback. Hopefully, this will go into the gr-osmosdr master at some point. It "works for me". Note that the gains on SDRplay are actually attenuations, so they appear backward. The LNA attenuation steps on RSPs are frequency dependent, so I gave up trying to make them look like gains. My main use case is GQRX, and that way was even more confusing. There is a GRC sdrplay source that is a little less generic than the osmocom source. There are probably ways to repackage this so it can be distributed freely, but I'll address that if there's more interest. The SDK include file is redistributable and its terms seem to be compatible with GPL. The shared object can be dynamically loaded over a placeholder library that does nothing. Yes, that's hackish and it would be better to have everything free.